1. Field of the Invention
The invention relates to a motor driven power steering system in which an auxiliary steering force is generated by an electric motor.
2. Description of the Prior Art
A power-assisted steering system for helping a driver to steer an automobile is well known. In a conventional power steering system, an auxiliary steering force is generated in accordance with the steering by a driver and the force thus generated is transmitted to a steering gear. Most of the power steering systems in practical use at the present time are actuated by a hydraulic drive arrangement. The hydraulic drive arrangement of conventional power steering systems is equiped with a control valve, hydraulic cylinder and related structure to generate an auxiliary steering force by moving or displacing oil or other hydraulic fluid within the cylinder in accordance with a steering operation.
One significant disadvantage of the above-described known hydraulic power steering systems, especially in view of the trend toward small cars, is that the control valve, hydraulic cylinder and other related structures, as discussed above, are large in size. Large pressure losses are also encountered within connecting lines of the arrangement. To prevent pressure losses, any bends in the connecting line must be with small curvature. However, in the case of a vehicle not having a large mounting space in the engine room, such as a front drive vehicle, these containts make it difficult to mount the power steering system. Also, in a conventional hydraulic drive unit, an effective seal must be used to ensure that oil does not leak. Additionally, handling of the system is difficult.
In order to avoid the above-identified problems, it has been proposed to use a motor as a driving device for a power steering system.
Referring now to FIG. 3, there is shown the general construction of such a conventional motor driven power steering system. A first steering shaft 42 is connected to a steering wheel 41, and a second steering wheel shaft 45 is connected to the first shaft 42 via a first universal joint 44. A third steering shaft 47 is connected to the second shaft 45 via a second universal joint 46. The bottom end of the third shaft 47 is connected to a pinion gear (not shown) engaged with a rack (not shown) for performing the steering of tires. The first shaft 42 has a torque sensor 48 fixedly mounted thereto. A DC servo motor DM is connected to the third shaft 47 via reduction gearing 49. The output of torque sensor 48 is connected to a control apparatus 50, the output of which is connected to the motor DM.
The operation in the conventional motor driven power steering system is as follows:
During a steering operation, the torque sensor 48 detects torque applied to the first shaft 42. The output of torque sensor 48 is transmitted to the control apparatus 50. The control apparatus 50 operates to supply an electric current to the motor DM in accordance with both the magnitude and direction of the torque. The motor DM is rotated in accordance with the electric current. The rotational speed of motor DM is reduced gearing 49.
Thus, in the above-described motor driven power steering system, the rotation of an auxiliary motor for generating the auxiliary steering force is transmitted to the steering shaft by a reduction gearing system. In general, the reduction gearing may be formed with a gear engagement involving a plurality of gears.
Such reduction gearing engagement has a certain gears backlash due to an aspect of manufacturing accuracy of the gearing wherein a clearance exists between intermeshing gear teeth. Therefore, when the motor rotates so as to produce the auxiliary steering force, the clearence is closed as the gear teeth make contact. As a result, a shock occurs at the moment when gears engage each other. This shock reduced the driver's feel of the power steering effect.